Field of the Invention
The invention relates to a method for fabricating a flexible printed circuit board which has an access on both sides.
Flexible printed circuit boards or so-called xe2x80x9cflexboardsxe2x80x9d are used as electrical connection elements and circuit carriers in a wide variety of technological areas.
The use of a cost-effective roll-to-roll process for producing flexible printed circuit boards with access on one side, i.e. with contact-making cutouts either only in the base sheet or only in the covering sheet, is already known. Base material, which is supplied in the form of a roll by the manufacturer of the base material and which includes a base sheet with a Cu sheet already applied thereto, is unwound continuously from the roll. The Cu sheet is patterned into conductor tracks and a covering sheet is rolled onto the patterned Cu sheet. The covering sheet is already provided with stamped-out portions at the contact-making locations that are to be formed subsequently. The material web thus fabricated is wound up onto a roll again at the end of the production line.
Printed circuit boards with access on both sides, i.e. with contact-making cutouts both, in the base sheet and in the covering sheet, cannot be fabricated in a roll-to-roll process, since this process, due to different expansion properties of the base material (base sheet with Cu sheet) and of the stamped covering sheet, does not have a sufficient positioning accuracy for the positionally highly accurate alignment of the stamped base film with the stamped covering film necessary in the case of a printed circuit board with access on both sides. For this reason, flexible printed circuit boards with access on both sides are not fabricated on a continuous material but rather in a comparatively cost-intensive method using individual-processing material sections or so-called xe2x80x9cpanels.xe2x80x9d
German Published, Prosecuted Patent Application DE-AS 19 25 745 discloses a method for fabricating, in a roll-to-roll process, a flexible printed circuit board with access on one side.
U.S. Pat. No. 5,759,417 discloses a process for fabricating a flexible printed circuit board which has access openings on both sides. In this case, a base material which is in the form of a sheet and which is coated on both sides with Cu is used. A conductor track pattern is formed from one of the Cu layers, while fine holes are introduced into the other Cu layer at suitable locations through the use of a photolithographic step. The conductor track pattern formed from the first Cu layer is covered with a covering sheet having stamped-out portions at suitable locations. On the other side of the printed circuit board, the perforated Cu layer is used as a mask layer for a laser processing step, in which contact-making holes are burned into the base material. Access to the Cu layer forming the conductor track pattern is also enabled via the contact-making holes. The Cu mask layer is removed in a final step.
It is accordingly an object of the invention to provide a method for fabricating a flexible printed circuit board with access on both sides which overcomes the above-mentioned disadvantages of the heretofore-known methods of this general type and which is cost-effective to carry out and allows a high flexibility in the process control.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for fabricating a flexible printed circuit board, which includes the steps of:
providing a base material having a base sheet and a metallic conductor track sheet disposed on the base sheet;
patterning the metallic conductor track sheet for producing conductor tracks;
providing a conductor track covering over the conductor tracks and providing a first contact-making cutout in the conductor track covering;
producing a second contact-making cutout in the base sheet with a laser irradiation by guiding, with an optical configuration, a beam of a laser, focused to a small-area laser light spot, along a predefined travel path across the base sheet and removing the base sheet in a region of the second contact-making cutout; and
performing the steps of patterning the metallic conductor track sheet, providing the conductor track covering, and producing the second contact-making cutout in at least one roll-to-roll process.
In other words, in accordance with the invention, a method for fabricating a flexible printed circuit board is provided, in which, on a base material including a base sheet and a metallic conductor track sheet provided thereon, in one or more roll-to-roll processes,
the conductor track sheet is patterned for the purpose of producing conductor tracks, and
a conductor track covering, in particular a covering sheet, is provided over the conductor tracks, first contact-making cutouts being provided in the conductor track covering, wherein the method is characterized in that
in the context of a roll-to-roll process, using laser irradiation, second contact-making cutouts are produced in the base sheet, by a laser beam, focused in a small-area, being guided by an optical configuration along a predefined travel path across the base sheet to be removed in the region of the second contact-making cutout to be produced.
With the objects of the invention in view there is also provided, a method for fabricating a flexible printed circuit board, which includes the steps of:
providing a base material having a base sheet and a metallic conductor track sheet disposed on the base sheet;
patterning the metallic conductor track sheet for producing conductor tracks;
providing a conductor track covering over the conductor tracks;
producing a first contact-making cutout in the conductor track covering and producing a second contact-making cutout in the base sheet with a laser irradiation by in each case guiding, with an optical configuration, a beam of a laser, focused to a small-area laser light spot, along a predefined travel path across the base sheet and the conductor track covering and removing the base sheet and the conductor track covering in regions of the first and second contact-making cutouts; and
performing the steps of patterning the metallic conductor track sheet, providing the conductor track covering, and producing the first and second contact-making cutouts in at least one roll-to-roll process.
A significant advantage of both methods according to the invention is that a cost-effective roll-to-roll process is used in the production of the flexible printed circuit board with access on both sides. The integration of the laser processing step into the roll-to-roll process, which uses a continuous material web, results in a maximum cost saving, since all the first and second contact-making cutouts in the printed circuit board layer structure have already been completed at the end of the cost-effective roll-to-roll process. Furthermore, in comparison with conventional methods which use a mechanical stamping step in order to form the second contact-making cutouts, both methods according to the invention have a high adaptabilityxe2x80x94which can be achieved in particular by the use of softwarexe2x80x94with regard to changes in the process configuration or process planning. Whereas a change in the desired stamping positions or patterns necessitates an exchange of parts of e.g. the stamping tool in the case of a conventional stamping process. In the case of the methods according to the invention on the other hand, only laser and/or laser positioning parameters have to be adjusted by software in such a case.
In accordance with the first method according to the invention, the laser processing step is preferably carried out after the application of the conductor track covering containing the first contact-making cutouts. In this case, it is appropriate to perform the process steps of patterning the conductor track sheet and providing or applying the conductor track covering, in a roll-to-roll process, on a continuous material web.
In accordance with the second method according to the invention, a laser processing step is used both, to produce the first contact-making cutouts and to produce the second contact-making cutouts. As in the case of the first method according to the invention, the two laser processing steps are carried out (if appropriate at different process instants) in the context of a roll-to-roll process in this case, too.
The effect achieved by guiding the laser beam, which is focused in a small area, along a predefined travel path is that only material in a small area is removed at each instant. As a result, the risk of damage to the base sheet material (conductor track covering material) is reduced, and the material is removed in a highly defined manner with good lateral control.
The laser preferably produces a light spot having a diameter in the range of 100-300xcexcm on the base sheet and, in the case of the second method, also on the conductor track covering.
A further preferred measure of the invention is characterized in that the light spot of the laser sweeps over the base sheet to be removed, and, if appropriate, also over the conductor track covering, with a velocity in the range of 10-50 cm/s, in particular about 20 cm/s. This ensures that, on the one hand, a sufficiently rapid production of the second contact-making cutouts and, if appropriate, also the first contact-making cutouts is achieved. On the other hand, the metallic conductor track sheet cannot be damaged by the laser light radiation being coupled in at the same location for an excessively long period of time.
Likewise, for reasons of affording protection against damage, the base sheet, if appropriate also the conductor track covering, is removed in layers in the region of the second contact-making cutout and, if appropriate, also in the region of the first contact-making cutout to be uncovered, by being swept over repeatedly, in particular from 2 to 4 times, by the laser light spot.
On accordance with another mode of the invention, the laser light spot is guided along a spiral travel path or a meandering travel path across the conductor track covering for producing the first contact-making cutout.
On accordance with a further mode of the invention, the laser light spot is guided along a spiral travel path or a meandering travel path across the base sheet for producing the second contact-making cutout.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for fabricating a flexible printed circuit board with access on both sides, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.